1. Field of the Invention
The present invention relates to a method for growing a compound semiconductor including a group III-V compound semiconductor layer which includes nitrogen and a group V element other than nitrogen as a group V composition; a quantum well structure including a group III-V compound semiconductor layer which includes nitrogen and a group V element other than nitrogen as a group V composition: and a compound semiconductor device including such a quantum) well structure.
2. Description of the Related Art
Recently, as group III-V compound semiconductor materials having a significantly wider field of use, group III-V compound semiconductor materials including a group V element other than nitrogen (arsenic (As), phosphorus (P), and antimony (Sb), etc.) and about several percents of nitrogen as a group V composition have been proposed. Nitrogen and group V elements other than nitrogen are significantly different from each other in atom diameter and electronegativity as described below. Due to such a difference, specific physical properties are generated by mixing nitrogen and a group V element other than nitrogen. The atom diameter is 0.070 nm for nitrogen: whereas it is 0.118 nm for arsenic, 0.110 nm for phosphorus, and 0.136 nm for antimony. The electric negativity is 3.5 for nitrogen; whereas it is 2.4 for arsenic, 2.5 for phosphorus, and 2.1 for antimony. For example, GaInNAs having a nitrogen composition ratio of several percents is considered to be obtained by mixing GaInAs and GaInN which has a larger forbidden band width than GaInAs, the GaInN being mixed at a ratio of several percents. However, GaInNAs having a nitrogen composition ratio of several percents has very large bowing on the change of the forbidden band width accompanying the mixing. Accordingly, such GaInNAs has the forbidden band width rapidly narrowed by the mixing although GaInN has a large forbidden band width.
The other physical properties of GaInNAs-based materials, such as refractive index, exhibit a specific behavior of significantly changing when a small amount of nitrogen is mixed. GaInNAS-based materials thus obtained have been found to be the only materials which can be used in a light emitting layer of a light emitting device which emits light having a wavelength of 1.3 xcexcm or 1.55 xcexcm (both of which are important for optical fiber communication) or a longer wavelength while being lattice-matched to a GaAs substrate, which is of high quality at low-cost. Accordingly, GaInNAs-based materials have recently become the target of attention industrially as materials to be used for a light emitting device.
By combining a group III-V compound semiconductor material, such as GaInNAs, including nitrogen and a group V element other than nitrogen with another group III-V compound semiconductor material having approximately the same lattice constant (for example, GaAs, AlGaAs, or InGaAsP), a hetero-junction having a very large band discontinuity (xcex94Eo) in the valence band can be formed. Therefore, it is predicted that a light emitting device including a light emitting layer formed of GaInNAs efficiently confines electrons injected into the light emitting layer even at a high temperature, and thus has a sufficiently small change in light emitting characteristics depending on temperature.
The hetero-junction is formed by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). The hetero-junction can also be formed by gas source molecular beam epitaxy (GS-MBE), metal organic molecular beam epitaxy (MO-MBE), chemical molecular beam epitaxy (CBE) or the like.
FIG. 11A shows a quantum well structure 1100 including an AlGaAs lower barrier layer 1101, a GaInNAs well layer 1102, and an AlGaAs upper barrier layer 1103 as a conventional example of a hetero-junction of compound semiconductors (conventional example 1). FIG. 11B shows supply sequences of sources for producing the quantum well structure 1100. In the example shown in FIG. 11B, the compound semiconductor layers are grown by MOCVD, using trimethyl gallium (TMGa), trimethylaluminum (TMAl), trimethyl indium (TMIn), arsine (AsH3), and dimethylhydrazine (DMeHy) as sources of Ga, Al, In, As and N, respectively. As a carrier gas, hydrogen (H2) is used. In FIG. 11, parts (a) through (e) show the supply sequences of the respective sources.
The AlGaAs lower barrier layer 1101 is grown in step M. Then, in step N, only AsH3 is supplied to suppress the vaporization of As, thereby pausing the growth. In this specification, a process of pausing growth will be referred to as a xe2x80x9cgrowth pausexe2x80x9d. In step O, the GaInNAs well layer 1102 is grown. Then, in step P, only AsH3 is supplied, thereby performing a growth pause. In step Q, the AlGaAs upper barrier layer 1103 is grown. During the growth pause in steps N and P, an optimum supply amount of AsH3 for each of the lower barrier layer 1101, the well layer 1102, and the upper barrier layer 1103 is set. H2 as the carrier gas is supplied at a constant amount throughout the steps M through Q.
Japanese Laid-Open Publication No. 10-144611 (conventional example 2) discloses a supply sequence for suppressing the generation of a metamorphic layer at a hetero-interface of a hetero-junction of layers of different group V compositions. FIG. 12A shows an FET crystal 1200 in conventional example 2 (shown in Japanese Laid-Open Publication No. 10-144611, FIG. 1). The FET crystal 1200 includes a GAs buffer layer 1212, an AlGaAs buffer layer 1213, a GaAs buffer layer 1214, an undoped Ga0.8In0.2As channel layer 1215, an n-type Ga0.5In0.5P electron supply layer 1216, an n-type Al0.2Ga0.8As Schottky layer 1217, and an n-type GaAs cap layer 1218 which are sequentially laminated on a semi-insulating GaAs substrate 1211 in this order.
The FET crystal 1200 is an example of hetero-junction of As-based materials including As as the only group V element (i.e., the undoped Ga0.8In0.2As channel layer 1215 and n-type Al0.2Ga0.8As Schottky layer 1217) and a P-based material including P as the only group V element (i.e., the n-type Ga0.5In0.5P electron supply layer 1216).
FIG. 12B shows supply sequences of sources for forming the undoped Ga0.8In0.2As channel layer 1215, the n-type Ga0.5In0.5P electron supply laster 1216, and the n-type Al0.2Ga0.8As Schottky layer 1217. In the example shown in FIG. 12B, PH3 is used as a P source and AsH3 is used as an As source. In FIG. 12B, parts (a) through (c) shows the supply sequences of a group III element, PH3 and AsH3, respectively.
In step R, the As-based material (undoped Ga0.8In0.2As channel layer 1215) is grown. Then, in steps S through U, a growth pause is performed. In step V, the P-based material (n-type Ga0.5In0.5P electron supply layer 1216) is grown. Then, in steps W through Y, a growth pause is performed. In step Z, the As-based material (n-type Al0.2Ga0.8As Schottky layer 1217) is grown.
In the above-described supply sequences, the two growth pause processes each include three steps, i.e., the step of supplying only the group V element used for growing the layer in the immediately previous step (steps S and W), the step of supplying no material (steps T and X), and the step of supplying only the group V element used for growing the layer in the immediately following step (steps U and Y).
Japanese Laid-Open Publication No. 10-270798 (conventional example 3) discloses a technology aiming at suppressing the formation of a metamorphic layer at a hetero-interface of a hetero-junction of an AlGaAs layer and a GaInNAs layer. FIG. 13 shows a semiconductor light emitting device 1300 in the conventional example 3 (shown in Japanese Laid-Open Publication No. 10-270798; FIG. 2). The semiconductor light emitting device 1300 includes an n-type GaAs buffer 1302, an n-type AlGaAs cladding layer 1303, an AlGaAs guide layer 1304, a GaAs spaces layer 1305, a GaInNAs well layer 1306, a GaAs spacer layer 1307, an AlGaAs guide layer 1308, a p-type AlGaAs cladding layer 1309, a p-type GaAs cap layer 1310, and an insulating layer 1312 which are sequentially laminated on an n-type GaAs substrate 1301 in this order. An n-type electrode 1313 is provided on the n-type GaAs substrate 1301, and a p-type electrode 1311 is provided on the insulating layer 1312.
In general, an AlGaNAs or AlGaInNAs metamorphic layer having an inferior surface state is provided at a hetero-interface between an AlGaAs layer and a GaInNAs layer. In conventional example 3, in order to avoid the generation of the metamorphic layer, the spacer layers 1305 and 1307 are respectively provided between the AlGaAs guide layer 1304 and the GaInNAs well layer 1306 and between the AlGaAs guide layer 1308 and the GaInNAs well layer 1306. The spacer layers 1305 and 1307 are each formed of a thin film having a thickness corresponding to at least one molecule.
Conventional example 3 discloses a hetero-junction structure but does not include any specific description on a method for growing a compound semiconductor, such as supply sequences of sources for compound semiconductor materials.
As described above, superior characteristics such as a characteristic temperature are predicted to be exhibited when a group III-V compound semiconductor layer (e.g., GaInNAs layer) including nitrogen and a group V element other then nitrogen as a group V composition for an active layer of a semiconductor laser device. However, when the above-described conventional methods for growing a compound semiconductor are used to form a quantum well structure in an active layer of a semiconductor laser device, a quantum well formed of GaInNAs including only about 1% of nitrogen does not necessarily provide superior light emitting characteristics to a quantum well formed of GaInNAs including no nitrogen. By contrast, by mixing only about 1% of nitrogen into the well layer, the oscillation threshold current is increased several to several tens of times and the light emission efficiency is reduced several to several tens of times. Group III-V compound semiconductor materials including nitrogen and a group V element other than nitrogen as a group V composition has specific characteristics which are not found in the other materials in the growth mechanism as well as in the physical properties. It is considered to be necessary to select a growth method compatible with the specific characteristics in the growth mechanism in order to produce a crystal and a quantum well structure having sufficient optical gain.
According to one aspect of the invention, a method for growing a compound semiconductor includes a first formation step of forming a first group III-V compound layer; a second formation step of forming a second group III-V compound layer including nitrogen and at least one group V element other than nitrogen as a group V composition, and a third formation step of forming a third group III-V compound layer between the first group III-V compound layer and the second group III-V compound layer, the third group III-V compound layer being formed for controlling a reactivity of the second group III-V compound layer with a nitrogen source used in the second formation step.
In one embodiment of the invention, the first formation step, the third formation step and the second formation step are performed sequentially in this order.
In one embodiment of the invention, the second formation step is performed continuously after the third formation step.
In one embodiment of the invention, the second formation step, the third formation step and the first formation step are performed sequentially in this order.
In one embodiment of the invention, the third formation step is performed continuously after the second formation step.
In one embodiment of the invention, the first formation step, the third formation step and the second formation step are performed sequentially in this order, and then the third formation step and the first formation step are performed sequentially in this order.
In one embodiment of the invention, the first formation step, the third formation step, the second formation step, the third formation step and the first formation step are continuously performed.
In one embodiment of the invention, a composition of the third group III-V compound layer is determined so that a reactivity of the third group III-V compound layer with the nitrogen source and a reactivity of the second group III-V compound layer with the nitrogen source are substantially equal to each other.
In one embodiment of the invention, the first group III-V compound layer includes at least one of aluminum and indium as a group III composition.
In one embodiment of the invention, the third group III-V compound layer includes at least one group V element other than nitrogen as a group V composition, and the third group III-V compound layer has a group III composition which is substantially equal to a group III composition of the second group III-V compound layer.
In one embodiment of the invention, the third group III-V compound layer has a group III composition ratio which is substantially equal to a group III composition ratio of the second group III-V compound layer.
In one embodiment of the invention, the second group III-V compound layer and the third group III-V compound layer each include indium as a group III composition, and an indium composition ratio of the third group III-V compound layer is in the range of xe2x88x9250% to +50% of an indium composition ratio of the second group III-V compound layer.
In one embodiment of the invention, the second group III-V compound layer and the third group III-V compound layer each include aluminum as a group III composition, and an aluminum composition ratio of the third group III-V compound layer is in the range of xe2x88x9230% to +30% of an aluminum composition ratio of the second group III-V compound layer.
In one embodiment of the invention, the third group III-V compound layer has a thickness which corresponds to one molecule at a minimum and a critical layer thickness at a maximum.
In one embodiment of the invention, the nitrogen source used in the second formation step is a compound expressed by the formula: 
where R1, R2, R3 and R4 are each a hydrogen atom or an arbitrary alkyl group.
According to another aspect of the invention, a quantum well structure produced by any one of the above-described methods is provided. The first group III-V compound layer is a barrier layer and a second group III-V compound layer is a well layer.
According to still another aspect of the invention, a quantum well structure includes a barrier layer including a first group III-V compound layer, a well layer including a second group III-V compound layer including nitrogen and at least one group V element other than nitrogen as a group V composition; and an intermediate layer including a third group III-V compound layer between the barrier layer and the well layer. The third group III-V compound layer includes at least one element other than nitrogen as a group V composition, and the third group III-V compound layer has a group III composition which is substantially equal to a group III composition of the second group III-V compound layer.
In one embodiment of the invention, the third group III-V compound layer has a group III composition ratio which is substantially equal to a group III composition ratio of the second group III-V compound layer.
According to still another aspect of the invention, a compound semiconductor device including at least the above-mentioned quantum well structure is provided. The quantum well structure acts as a light emitting layer.
According to still another aspect of the invention, a compound semiconductor device includes a first group III-V compound layer; a second group III-V compound layer including nitrogen and at least one group V element other than nitrogen as a group V composition, and a third group III-V compound layer provided between the first group III-V compound layer and the second group III-V compound layer. The third group III-V compound layer includes at least one element other than nitrogen as a group V composition, and the third group III-V compound layer has a group III composition which is substantially equal to a group III composition of the second group III-V compound layer.
In one embodiment of the invention, the third group III-V compound layer has a group III composition ratio which is substantially equal to a group III composition ratio of the second group III-V compound layer.
In one embodiment of the invention, the second group III-V compound layer and the third group III-V compound layer each include indium as a group III composition, and an indium composition ratio of the third group III-V compound layer is in the range of xe2x88x9250% to +50% of the indium composition ratio of the second group III-V compound layer.
In one embodiment of the invention, the second group III-V compound layer and the third group III-V compound layer each include aluminum as a group III composition, and an aluminum composition ratio of the third group III-V compound layer is in the range of xe2x88x9230% to +30% of the aluminum composition ratio of the second group III-V compound layer.
A method for growing a compound semiconductor according to the present invention grows an intermediate layer between a layer formed of an AlGaAs-based material, an InGaAsP-based material or the like and a layer including both nitrogen and a group V element other than nitrogen as a group V composition (for example, a GaInNAs layer, an InGaAsPN layer, a GaAsSbN layers or the like). The intermediate layer controls the reactivity of the layer to be grown thereon with a nitrogen source.
The present inventors have found that when a layer including both nitrogen and a group V element other than nitrogen as a group V composition is grown using a nitrogen compound expressed by the formula below as a nitrogen source, the decomposition and adsorption efficiency of the nitrogen source is significantly influenced by the group III composition, the group V composition, the composition ratios, the materials of the layer to be grown, and the materials of a layer below this layer. Based on this finding, the present inventors have provided an intermediate layer to uniformize the nitrogen concentration in the layer to be grown and reduce the generation of the non-light emission centers at a hetero-interface between the layer to be grown and the intermediate layer. As a result, the light emission intensity is improved. 
where R1, R2, R3 and R4 are each a hydrogen atom or an arbitrary alkyl group.
Aluminum exhibits a remarkably high reactivity with nitrogen sources, and indium is inactive with respect to nitrogen sources. Accordingly, the effect of the present invention is especially noticeable when either one of the layer to be grown or the layer below this layer includes aluminum or indium.
Thus, the invention described herein makes possible the advantages of providing a method for growing a compound semiconductor, including a group III-V compound semiconductor layer which includes nitrogen and a group V element other than nitrogen as a group V composition, which significantly improves the light emission characteristics and also is effective for forming a hetero-junction of a group III-V compound semiconductor layer which includes nitrogen and a group V element other than nitrogen as a group V composition and a group III-V compound semiconductor layer which does not include nitrogen. The invention described herein also makes possible the advantages of providing a quantum well structure having superior optical gain and superior light emission efficiency, by including a group III-V compound semiconductor layer which includes nitrogen and a group V element other than nitrogen as a group V composition as a well layer and a group III-V compound semiconductor layer which does not include nitrogen as a barrier layer; a method for growing a compound semiconductor which is preferable to form such a quantum welt structure, and a compound semiconductor device including the quantum well structure as an active layer.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.